ALTERATION OF THE EARLIER ANDESITE. 227 



of this earlier andesite has as yet been found in Tonopah. The last stage of 

 alteration in the rock (in the vein zones) was then in a sense the first work of 

 the waters, and the first stage, remote from the main circulation zones, the last; 

 and although the transition as studied is gradual, it by no means follows that 

 the rock near the veins went through all of the stages represented, but may 

 have reached its present condition much more directly. 



COMPOSITION OF MINERALIZING WATERS IN THE VEIN ZONES. 



In the unoxidized quartz veins the predominating gangue mineral is quartz, 

 with frequent adularia, subordinate muscovite (sericite), and comparatively rare 

 carbonates of lime, magnesia, manganese, and iron. The metallic minerals are 

 most prominently silver sulphide, containing sometimes antimony, arsenic, etc.; 

 silver selenide, gold in some form, copper-iron sulphide (chalcopyrite), iron 

 sulphide, and probably silver chloride. The mineralizing waters were then charged 

 with an excess of silica, and also probably, as the comparative analyses indicate, of 

 potash, together with silver, gold, antimony, arsenic, copper, lead, zinc, selenium, 

 etc. They were noticeably deficient in iron, since they have removed this metal 

 from the vein zones and the adjacent rock, more and more completely in propor- 

 tion as their work has been thorough, and the iron left in the veins is clearly a 

 residuum. That they contained carbonic acid and sulphur is shown by the for- 

 mation of sulphides and carbonates, not only in the veins but in the altered rock. 

 That they contained some chlorine and fluorine, though not in excessive amounts, 

 is indicated by the presence of a little original silver chloride and by their work 

 in forming muscovite, as will presently be explained. 



In the vein zone the maximum effect of these waters was a replacement of 

 nearly everything by precipitated silica. By a similar process of replacement 

 the sulphides, of which silver sulphide was the most prominent, were precipitated, 

 and the residue of the comparatively refractory iron was combined with free 

 sulphur to form pyrite. The residue of the comparatively refractor}- alumina 

 combined with the excessive silica and potash of the waters to form adularia 

 and muscovite (sericite). 



RELATION OF ADULARIA TO SERICITE AS ALTERATION PRODUCTS. 



It is necessary at the present point of the inquiry to investigate the conditions 

 of formation of adularia and of muscovite. Both are silicates of aluminum and 

 potassium, and both are conspicuous as secondary products in the altered andesite, 

 especially of the feldspar. The typical andesine-oligoclase alters sometimes to 

 adularia, sometimes to quartz and muscovite, sometimes to both. That one of these 

 products is not the alteration product of the other is shown by the fact that they 



